There is known a current sensor having a conductor (i.e., a current bar), through which a current to be measured flows, is enclosed with an annular magnetic core, and a hall element for the measurement of a magnetic field is arranged on a gap of the magnetic core. In such an electric power sensor, a position relationship between the magnetic core, the current bar and the hall element is fixed so that the magnetic core, the current bar and the hall element do not contact each other. In Japanese Laid-open Patent Publication No. 11-251167, the magnetic core divided into two upper and lower portions is stored in a housing through a core holder, so that the above-mentioned members are fixed.